1. Field of the Invention
The present invention relates to a multi-color display device capable of compressing color image data to store the compressed color image data in an image memory.
2. Description of the Related Art
Human beings recognize color and brightness with three types of visual cells each having different spectral sensitivity by converting external light into three types of electrical signals and determining the color and brightness based on the combination of the three types of electrical signals. According to the human visual characteristics, a color image display device displays a color image by combining three primary colors. Red (R), green (G), and blue (B) colors are used as the primary colors. Any combination of the three colors can be represented as a point located in a three-dimensional space. In general, specifying a point in the three-dimensional space requires three different types of signals.
To display a color image, three or more types of signals can be used. A white color can be used as a color that can be visually brightly viewed since white light includes all wavelengths of the visible spectrum. In addition, cyan and yellow colors can be used as the primary colors to be displayed.
JP-A-60-61724 discloses that white light is used for emission (or transmission) of light in a display device. A liquid crystal display generally includes sub-pixels (each of which is the minimum driving unit) having respective RGB color filters. When a sub-pixel (white sub-pixel) of a white color is added to the sub-pixels having the respective red, green and blue color filters, a color image is displayed with a combination of red, green, blue and white colors. Since the white sub-pixel does not include a color filter, it has high transmittance and serves to improve brightness. Specifically, when a conventional pixel composed of the red, green and blue sub-pixels and a pixel composed of the red, green, blue and white sub-pixels have the same area, the ratio of the area of each of the red, green and blue sub-pixels included in the conventional pixel to the area of each of the red, green, blue and white sub-pixels included in the pixel is 4:3. Each of the red, green and blue color filters cuts off two thirds wavelength components of light emitted by a light source, while the white sub-pixel transmits all components of light emitted by the light source. Based on the abovementioned relationships, the ratio of the maximum amount of light to be displayed on a RGB panel (including red, green and blue sub-pixels) to the maximum amount of light to be displayed on a RGBW panel (including red, green, blue and white sub-pixels) is ((4+4+4)/3):((3+3+3)/3+3×1)=1:1.5. In addition, JP-A-2005-138421 discloses a method for compressing data on a color image using a reduced memory capacity. JP-B-1994-7688 (Japanese Patent No. 1882816) discloses a technique for compressing data on a color image at a fixed compression rate which is not based on the content of the image.